Puffer interrupters have enjoyed ever increasing commercial success. This is due in part to their simple construction and excellent service record. The increased use of puffer interrupters in power class circuit breakers has been at the expense of the more complex two-pressure interrupters. Unfortunately, compared to a two-pressure interrupter, a puffer interrupter requires a relatively large prime mover.
The prime mover of a power class interrupter is a small part of the total cost of the interrupter relative to the cost of a prime mover in a distribution class or subtransmission class breaker or interrupter. Therefore, the design and cost of a relatively large prime mover impacts the cost of a power class circuit breaker to a lesser extent than a distribution or subtransmission class breaker. If one were to design a distribution class breaker or recloser using the puffer interrupter concept, the designer must therefore endeavor to minimize the energy consumed by the prime mover in operating the breaker.
The breaking process is characterized by an arc appearing for a limited period of time across the gap between the opening contacts of the breaker. See TRV and Interrupter Interaction, Parts I and II), by N. Holmgren, "The Line", Volume 78/4 and 79/1). This arc plasma column imposes severe environmental conditions on the components of the interrupter; for example:
a. the arc plasma has a temperature exceeding 20,000 degrees Kelvin;
b. the turbulent supersonic flow of the quenching gas in a changing flow geometry ranges from a few hundred meters per second to several thousand meters per second; and
c. the voltage and gradient placed upon the components in the vicinity of the arc is large (e.g. 10 KV/cm.)
Immediately after the current passes through a current zero, the critical stress is dependent upon the rate at which the recovery voltage rises. This rate is relatively high following a short-line fault (e.g. 2-7 Kv/.mu.sec). Those points at which the arc roots are located after contact separation are particularly high stressed and in addition are contaminated with metalic vapor and ionized fluid. Typically, these arc by-products are deionized and removed by a concentrated gas blast in the vicinity of the arc. Because the gap region is open to the interior surface of the interrupter housing, part of the arc by-products are dissipated to the surrounding gas, while the majority is drawn through the tubes or hollow contact elements of the puffer.
Normally, the nozzle surfaces across which the arc is formed "ablate" during the interruption process. As a nozzle ablates, its dimensions change and the arc geometry is effected. Ultimately, with the deterioration of the nozzle surfaces, the interruption rating of the device will be effected. Thus, the service life of a puffer interrupter will be increased by having components in the vicinity of the arc that effect the geometry or the flow of arc extinguishing fluid resistant to ablation by the arc.
In addition to the ablation problem, those skilled in the art known that materials in the gap having a dielectric constant different from that of the gas or arc extinguishing fluid cause a distortion in the surrounding potential field. This distortion can cause high-voltage stresses to appear across the gap. These high-voltage stresses, in turn, can initiate a flash over of the contacts while they are separated or when the interrupter is opened.
If the materials having a different dielectric constant (i.e., so called "shunting dielectrics") are removed, a more uniform potential field is created in the vicinity of the gap. This in turn would reduce the voltage stress and decrease the possibility of a flashover. In this context "shunting dielectrics" include such materials as TEFLON (polytetrafluoroethylene) which have a dielectric constant significantly different from that of the quenching gas (i.e., typically, sulphurhexafluoride SF.sub.6).
Areas of low dielectric strength can also be reduced by increasing the circulation of gas within the interrupter. Increased gas circulation would prevent stagnant gas (i.e., the gas most recently involved in the interruption process) from remaining concentrated in any one particular area of the interrupter. Circulation would provide a "mixing action" which would insure that the arc extinguishing gas has a more uniform dielectric strength throughout the interior of the interrupter. The gas most recently involved in the interruption process has a lower overall dielectric strength. Therefore, it is important to insure that gas having a relatively low dielectric strength does not build up in regions such as that surrounding the open contact gap and any point where a significant voltage stress exists between the current carrying parts of the interrupter and the ground. Critical insulating surfaces are those insulating surfaces in close proximity to the arc blast. Thus, if these surfaces are shielded from the arc, the fall out of arc products is minimized and the surrounding insulating surfaces can withstand the high-intensity radiation from the arc without the danger of a restrike or flashover.
Other inventors have recognized the problems associated with the interruption process; few however have proposed an apparatus that efficiently and economically resolves the problem of voltage stress and gas circulation. Hanke (in U.S. Pat. No. 4,146,763) employed an annular electrode positioned adjacent the end portion of the blasting cylinder in a puffer interrupter to act as a collecting electrode to "even-out" the charge distribution on the blasting cylinder. Fischer (in U.S. Pat. No. 4,249,049) disclosed a plane break circuit interrupter incorporating a set of moving and stationary contact shields which surround the main contacts of the interrupter. This arrangement is thought to prevent the formation of critical points of dielectric stress, both during the opening and the closing cycle of the interrupter. Thus, when closing, pre-strike should be prevented and when opening restrike and reignition should be prevented. Finally, Gonek (in U.S. Pat. No. 4,086,461) uses an external moveable bridging contact comprising an open-ended, cylindrical sleeve of electrically-conductive material to direct the flow of arc extinguishing gas and to improve the dielectric withstand to high-voltage (i.e., switching and lightning overvoltages).
For the most part, each of these earlier designs is relatively complicated, requires extensive mechanical linkages and connections, and is expensive to manufacture and maintain in operation once it is put in use. It is especially significant that no one has thought of confining the arc extinguishing process within a plenum or chamber separately disposed from the housing which encloses the interrupter.
An inexpensive, relatively simple, innovative design for a small compact buffer interrupter of the subtransmission or distribution class variety would be a welcome addition to the art. This would be particularly true if the design incorporated features which would reduce maintenance and operating costs.